There have been numerous dramatic changes to Australia’s rivers since European settlement and these have collectively contributed to substantial declines in stocks of native fish, including iconic species such as the Murray cod. Overfishing from the mid 19th century is often regarded as a major driver of initial population declines, but subsequent destruction of habitat, alteration of river flows, and invasion by introduced species now means there are numerous competing hypotheses about the dominant factors constraining the recovery of populations toward their former levels. Despite large investments in river restoration and enhancement of native fish populations with accompanying empirical studies to determine the benefits from this investment, evidence of a large-scale recovery of populations is still absent.

A range of measures has been suggested to rehabilitate native fish populations, but to date most actions have focused on top down pressures such as demographic bottlenecks associated with habitat loss, barriers to movements, and angling pressure. In contrast, ‘bottom-up’ processes that result in energetic limitations have not been explored in detail. One widely held, but currently unsubstantiated, view is that feeding lower in the food chain by detritivore species such as carp has effectively redistributed (or ‘locked away’) energy once available to predatory native fish such as cod. Similarly, altered flooding regimes may have reduced the total pool of energy available to drive fish production in many lowland rivers. Such ecosystem-type impacts on production and food chain energetics are fundamental to our understanding of ecosystem function, but rarely can be considered in ‘operational’ type funding arrangements.

This working group will bring together ecologists with expertise in fish and floodplain ecology and NRM managers to explore the potential for these different but overarching mechanisms to constrain the recovery of native fish stocks. We will combine early historical and current data on fish stocks with simple demographic and bio-energetic models to help quantify the relative merit of different hypotheses regarding possible recovery constraints. The working group outputs will improve our knowledge of energy flows through aquatic food-webs and help to better understand the merits of targeting different drivers in restoring native fish populations.

The first of the “where have all the fish gone” working group’s two meetings was held on Stradbroke Island in November 2012. The meeting brought together freshwater fish ecologists working on a range of systems – from heavily regulated rivers in the Murray-Darling Basin to the free-flowing rivers of the northern tropics, and with interests ranging from historical ecology to eco-physiology. The overall aim of our working group is to examine historical declines in native fish abundance in heavily modified river systems from an ecosystem perspective – focusing on changes in basal production rates, altered energy pathways and potentially reduced carrying capacity.

We spent much of the first day discussing the sorts of assumptions that have underlain much of the research on native-fish population dynamics, highlighting numerous knowledge gaps regarding the role of density dependent processes (e.g. altered basal resource availability, competition with invasive species) in regulating fish abundances. Considerable effort went into identifying a range of possible hypotheses that might explain historical declines associated with human disturbances, so that these could be explored in a logical sequence, rather than simply trying to pursue the one hypothesis with strongest support. The sequence builds on an increasing set of assumptions and data requirements and thus should also prove an efficient line of enquiry.

We then set about identifying appropriate case studies and the necessary datasets and information needs. It was agreed that the Murray River was an appropriate case study because of the strength of both contemporary and historical information on fish abundances, and well documented changes in flooding regimes, which are major drivers of production in floodplain rivers.

Our second day was broken up by a midday swim in Blue Lake before heading back to work in smaller groups. One group focused on developing food webs for large lowland rivers to identify the pathways linking basal primary production and detrital breakdown to higher order predators such as Murray Cod, which historically dominated fish biomass in the Murray. Another group of lateral thinkers developed a suite of graphical conceptual models to support the conceptual models outlined on day 1. A third group discussed modelling approaches, and how to best integrate uncertainty and a range of data sources.

By the end of the meeting numerous bits of homework had been identified and allocated to sub-groups within the team. At the next meeting we expect to have developed estimates of historical cod population sizes from commercial fisheries records from the mid and lower Murray River, derived estimates of historical and contemporary production rates (from primary producers and detrital breakdown) based on historical flooding regimes, and developed prototype models to link these data via simple food webs.

Following our first successful “where have all the fish gone” workshop in October 2012, the group reconvened on Stradbroke island in April 2013. We also welcomed into the group Darren Baldwin (MDFRC) and Keller Kopf (Charles Sturt University). Their involvement provided a valuable complement to the range of expertise within the group and we spent a considerable block of time discussing the challenges of scaling up estimates of basal production, especially from floodplain sources. As with the first workshop, the lively discussion quickly highlighted a range of knowledge gaps around competitive interactions, the importance of food quality, and flux pathways (biotic and abiotic) of floodplain derived energy sources.

Prior to this second meeting we had made considerable progress in quantifying the food web and developing the modelling framework for our case study system – the Murray River. While the modelling is not yet complete, some of the background work on scaling up estimates of algal production clearly demonstrates that additional energy sources (nominally terrestrial inputs) are required to support contemporary fish biomass estimates. This provides quantitative support for our conceptual understanding of the role of floodplain-derived carbon in supporting in-stream food webs in large lowland rivers.

Given this was our last workshop, we devoted roughly half of our time to writing. Having agreed on an appropriate manuscript structure, we broke into small workgroups each focussing on a specific aspect of the manuscript. This turned out to be a very effective use of people’s time, and by the end of the meeting we had a sufficiently developed draft to ensure, firstly, that everyone’s efforts thus far were captured, and secondly, that we had a solid basis around which to maintain our interactions after the meeting.

Thus while we have some work ahead in tying up some lose ends and completing our scenario modelling, the team is well placed to answer our original questions about energetic constraints on fish carrying capacities.

The synthesis exercise that ACEAS has supported has provided fresh perspectives on how to link flow, habitat and fisheries management in lowland rivers, and forms a sound basis for further research.

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